Required Firewall Model Suggestion

Without more info, it's hard to say.  In general I recommend making it so east-west traffic flows through the FortiGate as well, so it's not just about what your ISP gives you (north-south).  Plus you have to consider where you'll be over the life of the unit.  How will your user count and bandwidth needs likely change over the next 5 years?

You *might* be able to get away with something as light as the 41F, or more likely 61F, but you may need the 101F or could even use the 201F if you see some significant growth on the horizon or have a lot of internal traffic.